Practical Skills - Organic Preps Flashcards
How do you calculate the density of a liquid?
density = mass / volume
check units!
How do you cool an organic liquid?
Use an ice bath
What is reflux?
Reflux allows the reactants (and products) to be heated but not escape from the reaction vessel (as heating causes them to evaporate or even boil). Any escaping vapour condenses and falls back into the flask.
Draw a diagram for heating by reflux
What do antibumping granules do?
Any bumping granules are placed in the flask to prevent bumping, (the sudden release of a large bubble of vapour that makes the reaction mixture jump up). The anti-bumping granules produces many small bubbles rather than a large one.
How would you purify a liquid?
Distill the reaction mixture collecting at the boiling point for the desired compound.
Draw a diagram of distillation
When separating out a solid from a solution why might you add ice to the solution
This reduces the solubility and so the solid will come out of soution and canbe collected
How do you collect a solid from a solution/liquid
Use a Buchner funnel to collect the solid under reduced pressure
Draw a diagram of a Buchner Funnel
How do you purify an impure solid?
Solids are purified by recrystallisation.
Describe how recrystallisation works.
The impure solid product is dissolved in the minimum amount of a hot solvent to create a solution saturated with the product.
Any insoluble impurities can be filtered out (but the apparatus must be heated so the solution does not cool as it is filtered).
The saturated solution is then allowed to cool slowly, and so the product crystallises out from the solution while any other impurities remain dissolved.
In recrystallisation why is the saturated solution cooled slowly?
If the saturated solution is cooled too quickly many of the impurities crystallise out as well.
In recrystallisation why is a minimum quantity of hot water was used?
To ensure the hot solution would be saturated and crystals would form on cooling.
In recrystallistion the flask is left to cool before crystals are filtered off. Why?
Solubility of the product will be higher if warm so the yield will be lower.
In recrystallisation the solid sample is collected and compressed into a Buchner funnel for filtering. Why is it compressed in the funnel?
Air will pass through the sample (rather than around it) and ensure better drying odf the sample.
In recrystallisation, before the sample is filtered to remove water a little cold water is poured through the crystals. Why do this?
To wash away soluble impurities
Why do we dissolve an impure sample in a solvent before doing recrystallisation?
So that any insoluble impurities can be removed
How do you purify a liquid?
Put it in a separating funnel with a solvent that does not mix with the
product, but in which impurities may dissolve.
How do you remove water from a liquid that you are trying to purify?
Add a chemical that absorbs water, such as anhydrous sodium sulphate or anhydrous calcium chloride.
After purifying a liquid by distillation and then adding other solvents to remove further impurities, how could you further purify a liquid?
Perform another distillation and then collect the product over a very narrow temperature range (e.g. 5°C).
How do you test the purity of a product?
See what temperature the product boils or melts at. The narrow the range the purer the product
If a substance melts over a road temperature range, what can you say about its purity?
It is impure
How do you measure the yield of a reaction?
